With reference to FIG. 1, a ducted fan gas turbine engine generally indicated at 10 has a principal and rotational axis X-X. The engine comprises, in axial flow series, an air intake 11, a propulsive fan 12, an intermediate pressure compressor 13, a high-pressure compressor 14, combustion equipment 15, a high-pressure turbine 16, and intermediate pressure turbine 17, a low-pressure turbine 18 and a core engine exhaust nozzle 19. A nacelle 21 generally surrounds the engine 10 and defines the intake 11, a bypass duct 22 and a bypass exhaust nozzle 23.
The gas turbine engine 10 works in a conventional manner so that air entering the intake 11 is accelerated by the fan 12 to produce two air flows: a first air flow A into the intermediate pressure compressor 13 and a second air flow B which passes through the bypass duct 22 to provide propulsive thrust. The intermediate pressure compressor 13 compresses the air flow A directed into it before delivering that air to the high pressure compressor 14 where further compression takes place.
The compressed air exhausted from the high-pressure compressor 14 is directed into the combustion equipment 15 where it is mixed with fuel and the mixture combusted. The resultant hot combustion products then expand through, and thereby drive the high, intermediate and low-pressure turbines 16, 17, 18 before being exhausted through the nozzle 19 to provide additional propulsive thrust. The high, intermediate and low-pressure turbines respectively drive the high and intermediate pressure compressors 14, 13 and the fan 12 by suitable interconnecting shafts.
In the development of gas turbine engines, it is important to determine the amount of vibration of the rotating blades. From vibration measurements, stresses induced in the blades may be determined. Action can then be taken to avoid stresses which are high enough to cause damage to the blades.
As described for example in US patent application no. 2002/0162395, it is known to mount strain gauges on rotating compressor/turbine blades to provide information about the amplitudes and frequencies of vibration of the blades. One or more strain gauges can be provided on each blade, and connected to a radio telemetry system mounted on the rotor, which transmits the measurements from the rotor. However, due to the number of strain gauges required to fully determine the vibrations, the telemetry system is typically complex, expensive, large and time-consuming to install within the rotor.
An alternative technique for characterising blade vibration is “blade tip timing” (BTT) in which non-contact timing probes (e.g. capacitance or optical probes), typically mounted on the engine casing, are used to measure the time at which a blade passes each probe. This time is compared with the time at which the blade would have passed the probe if it had been undergoing no vibration. This is termed the “expected arrival time” and can be calculated from the rotational position of the particular blade on the rotor in conjunction with a “once per revolution” (OPR) signal which provides information about the position of the rotor. The OPR signal is derived from the time at which an indicator on the rotor passes a reference sensor, and its use is well known in the art.
The difference between the expected arrival time and the actual arrival time can be multiplied by the blade tip velocity to give the displacement of the blade from its expected position. Thus BTT data from a particular probe effectively measures blade tip displacement at the probe.
Advantageously, the tip timing method does not require a telemetry system since the probes are mounted on the casing. However, because the sampling rate of the probes is determined by the rotational frequency of the rotor, it is often below the Nyquist frequency for the vibrations of interest. Thus each probe undersamples the vibrations, leading to problems such as aliasing. A further problem with BTT data is that it is often intrinsically noisy due to probe movement caused by mounting restrictions and casing thickness. Nonetheless, with a plurality of timing probes, it is possible, in principle, to perform useful vibration analysis that can be converted into blade stresses.
Conventionally BTT data is separated into two categories: synchronous, where the sample rate is an exact multiple of the signal data; and asynchronous where there is no direct correlation between sample rate and signal data. A synchronous response is an integer multiple of the rotational speed, and is locked to this over a small range of speeds. These are often termed engine order excitations.
Synchronous vibrations manifest themselves as DC shifts in blade position due to the relatively low sampling rate and the vibration occurring at integer multiples of the OPR signal. Synchronous vibrations can be particularly damaging to a rotor blade. A method of analysing BTT data that can be used for identifying resonant synchronous vibration events is described in EP 2199764.
Conventionally, BTT data has been processed, and in particular filtered, differently depending on whether a synchronous and asynchronous analysis is being performed. This can lead to problems with operator filter selection if the analysis is being performed off-line, or to an undesirable reliance on automatic and correct determination of whether a particular response is synchronous and asynchronous if the analysis is being performed online.
Thus it would be desirable to provide an approach for analysing BTT data which can be performed on-line and which does not require a determination of whether a response is synchronous or asynchronous.